Add Android font support

[paulproteus]

Add Android font support for text widgets (Label, NumberInput, TextInput) including bold & italic & font size.

This does not add fantasy or cursive support. Those are a smidge more complicated, and I'd prefer to add those in a separate pull request.

Screenshot of TravelTips now:

Feedback welcome.

Implementation choices

Sometimes, calls to Android measure() will throw NullPointerException. For some widgets, they need LayoutParams (which are only added in Toga when a widget gets added to a container) before measure() can be called. This does an explicit check for None in those cases, which does the job fine, so I propose we roll with it.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[paulproteus]

Cursive and Fantasy fonts aren't enabled here. They're not part of the Android Typeface class, although it looks possible to add support with decent reliability. I'm not 100% sure. If that's important, I'm happy to see an issue assigned to me.

If you have cosmetic/smallish changes to make, then feel very free to make them.

If there are larger or otherwise issues you think I should address, then I'm happy to.

Cheers!

[paulproteus]

I've added support for cursive!

You can see it working in this screenshot.

(I've modified the app to use cursive for one of the tip fields -- look for a lot of 6s, and one of those two fields is cursive.)

[freakboy3742]

Mostly looks good. I've made some small tweaks around the original font definition and the way font binding is handled; plus I've added font usage to multiline text.